SC185 datasheet(13/18 Pages) SEMTECH

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SC185

Applications Information

Detailed Description

The SC185 is a synchronous step-down Pulse Width

Modulated (PWM), DC-DC converter utilizing a 1.5MHz

fixed-frequency voltage mode architecture. The device is

designed to operate in fixed-frequency PWM mode and

has the option to enter power save mode (PSAVE) at light

loads to maximize efficiency. The switching frequency is

chosen to minimize the size of the external inductor and

capacitors while maintaining high efficiency.

Operation

During normal operation, the PMOS MOSFET is activated

on each rising edge of the internal oscillator. The period

is set by the onboard oscillator when in PWM mode. The

device has an internal synchronous NMOS rectifier and

does not require a Schottky diode on the LX pin. The

device operates as a buck converter in PWM mode with

a fixed frequency of 1.5MHz at medium to high loads. At

light loads, depending on the MODE pin configuration, the

part will either enter PSAVE mode to maximize efficiency

or stay in forced PWM mode.

Power Save Mode Operation

Connect the MODE pin to ground to enable the PSAVE

mode. When the load current decreases below the PSAVE

threshold, PWM switching stops and the device automati-

cally enters PSAVE mode. This threshold varies depending

on the input voltage and output voltage setting, optimiz-

ing efficiency for all possible load currents - whether in

PWM or PSAVE mode.While in PSAVE mode, output voltage

regulation is controlled by a series of bursts in switching.

During a burst, the inductor current is limited to a peak

value which controls the on-time of the PMOS switch. After

reaching this peak, the PMOS switch is disabled and the

inductor current is forced to near 0mA. Switching bursts

continue until the output voltage climbs to V

OUT +2% or

until the PSAVE current limit is reached. Switching is then

stopped to eliminate switching losses, enhancing overall

efficiency. Switching resumes when the output voltage

reaches the lower threshold of V

OUT and continues until the

upper threshold again is reached. Note that the output

voltage is regulated hysterically while in PSAVE mode

between V

OUT and VOUT + 2%. The period and duty cycle

while in PSAVE mode are solely determined by V

IN and VOUT

until PWM mode resumes. This can result in the switching

frequency being much lower than the PWM mode

frequency.

If the output load current increases enough to cause

OUT to decrease below the PSAVE exit threshold (VOUT -

4%), the device automatically exits PSAVE and operates

in continuous PWM mode. Note that the PSAVE high

and low threshold levels are both set at or above V

OUT to

minimize undershoot when the SC185 exits PSAVE. Figure

1 illustrates the transitions from PWM mode to PSAVE

mode and back to PWM mode.

Figure 1 — Transitions between PWM and PSAVE

Modes

Protection Features

The SC185 provides the following protection features:

Current Limit

Over-Voltage Protection

Soft-Start Operation

Thermal Shutdown

•

VOUT

VOUT +2.5%

VOUT -2%

Load

Demand

(

IOUT)

BURST

OFF

VLX

Time

PWM Mode at

Medium/High

Load

PSAVE Mode at

Light Load

PWM Mode at

Medium/High

Load

PSAVE

EXIT

VOUT +2%

VOUT - 4%

SC185 Datasheet(PDF) 13 Page - Semtech Corporation

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